Let’s denote the first term of the arithmetic sequence as a and the common difference as d.

We are given that the 3rd term is 5, so we can write the equation: a + 2d = 5

We are also given that the 5th term is 11, so we can write the equation: a + 4d = 11

Now we have a system of two equations with two variables. We can solve this system to find the values of a and d.

Subtracting the first equation from the second equation, we get: (a + 4d) - (a + 2d) = 11 - 5 2d = 6 d = 3

Substituting the value of d into the first equation, we get: a + 2(3) = 5 a + 6 = 5 a = -1

So the first term of the arithmetic sequence is -1 and the common difference is 3.

Now we can find the sum of S5, which represents the sum of the first 5 terms of the arithmetic sequence.

S5 = (5/2)(2a + (5-1)d) S5 = (5/2)(2(-1) + (5-1)(3)) S5 = (5/2)(-2 + 4(3)) S5 = (5/2)(-2 + 12) S5 = (5/2)(10) S5 = 25

Therefore, the sum of S5 of the arithmetic sequence is 25.